PURPOSE: To manufacture a duct provided with a flange by only blow molding, by a method whrein the title product possesses a fitting part comprised of the first resin substance provided with a fitting surface into which a through hole is formed and a blow-molded hollow part comprised of the second resin substance inserted into the through hole and integrated with the fitting part.

CONSTITUTION: An undercut forming part 2c is formed within a through hole of fitting parts 2 and the fitting parts 2 are fitted to the inside of grooves formed into a pair of molds 5a, 5b. A gap 6 is set up between the pair of the molds 5a, 5b. Parison 7 is introduced into the gap 6 between the pair of the molds 5a, 5b and inserted into through hole 2a of the fitting part 2. The pair of the molds 5a, 5b are clamped, compressed air is sucked into the parison and blow molding is performed. The parison is blow-molded into a predetermined form decided by a form of a cavity of the pair of the molds 5a, 5b and a blow-molded hollow part 7' is formed. An undercut coating part 7'c is formed after a form of the undercut forming part 2c in the blow-molded hollow part 7' and integrated with the fitting part 2.